This invention relates to an image reading apparatus for a facsimile or scanner, and more particularly to a light-source-incorporated image sensor, in which a light receiving element and an EL (Electro Luminescence) light emitting element are provided as one unit for the purpose of miniaturization, which is made large in focal depth.
Heretofore, a facsimile or scanner employs an image reading apparatus which comprises: a light source, namely, a fluorescent lamp; an image sensor; and an equi-magnification optical system for forming the image of an original on the image sensor. The apparatus is disadvantageous in that, since a rod lens array or the like is employed as the equi-magnification optical system, the apparatus is difficult to miniaturize.
In order to eliminate this difficulty, an extremely small light-source-incorporated image sensor has been proposed in the art which is obtained by providing an EL light emitting element and a close contact type image sensor as one unit.
The light-source-incorporated image sensor is formed as follows: That is, as shown in FIGS. 5 and 6, a film light receiving array 11 is formed on an insulating substrate made of glass or the like by using amorphous silicon, a transparent insulating layer 12 is formed on the light receiving element array 11 thus formed, and then an EL light emitting element 13 is formed on the transparent insulating layer 12. The EL light emitting element 13 is of a sandwich structure, comprising a metal electrode 14, an insulating layer 15a, a light emitting layer 16, an insulating layer 15b, and a transparent electrode 17 which are formed one on another in the stated order. As shown in FIG. 6, the metal electrode 14 has a plurality of light incident windows 14a respectively in correspondence to light receiving portions 11a of the light receiving element array 11. The output light of the light emitting layer 16 is reflected from an original 100 placed on the side of the transparent electrode 17 so that it is applied through the light incident windows 14a to the light receiving portions 11a of the light receiving element array 11 (cf. Japanese Patent Unexamined Publication No. Sho. 59-210664).
In the light-source-incorporated image sensor thus constructed, the output light of the EL light emitting element 13 is applied to the surface of the original 100, and the light reflected from the surface of the original is applied to the light receiving element array 11. Hence, when the area of the light incident windows 14a of the metal electrode 14 is in a predetermined range, then the focal depth of the image sensor depends greatly on the distance d.sub.1 between the original 100 and the light incident windows 14a of the metal electrode 14, and on the distance d.sub.2 between the light receiving element array 11 and the light incident windows 14a of the metal electrode 14.
If the focal depth is small or shallow, then the resolution (MTF) of the image sensor is considerably decreased in the case where the original floats above the predetermined position, or in the case where the original itself is uneven (as in the case of a PPC original on which a thick toner layer is formed) or in the case where a thin original is read which is inserted into a document carrier made of transparent films. Accordingly, in these cases, the image reading operation is not satisfactorily carried out.
With the above-described conventional image reading apparatus, it is difficult to adjust the distance d.sub.1 between the original 100 and the light incident windows 14a, and the thickness d.sub.2 of the transparent insulating layer 12 to desired values. Accordingly, it is also difficult to obtain a light-source-incorporated image sensor improved in focal depth.